Calcium Phosphate Bioceramics with Tailored Crystallographic Texture for Controlling Cell Adhesion
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Calcium Phosphate Bioceramics with Tailored Crystallographic Texture for Controlling Cell Adhesion Hyunbin Kim1, Renato P. Camata2, Sukbin Lee3, Gregory S. Rohrer3, Anthony D. Rollett3, Kristin M. Hennessy4, Susan L. Bellis4, and Yogesh K. Vohra2 1 Department of Materials Science and Engineering, University of Alabama at Birmingham, Birmingham, AL, 35294 2 Department of Physics, University of Alabama at Birmingham, Birmingham, AL, 35294 3 Department of Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, PA, 15213 4 Department of Physiology and Biophysics, University of Alabama at Birmingham, Birmingham, AL, 35294 ABSTRACT The orientation distribution of crystalline grains in calcium phosphate coatings produced by pulsed laser deposition was investigated using an X-ray pole-figure diffractometer. Increased laser energy density of a KrF excimer laser in the 4–7 J/cm2 range leads to the formation of hydroxyapatite grains with the c-axis preferentially aligned perpendicularly to the substrates. This preferred orientation is most pronounced when the plume direction of incidence is normal to the substrate. This crystallographic texture of hydroxyapatite grains in the coatings is associated with the highly directional and energetic nature of the ablation plume. Anisotropic stresses, transport of hydroxyl groups, and dehydroxylation effects during deposition all seem to play important roles in texture development. Studies of mesenchymal stem cell/biomaterial interactions show that the surfaces with an oriented distribution of hydroxyapatite grains promote significantly better cell adhesion than surfaces with random grain distribution. INTRODUCTION The excellent biocompatibility and bioactivity of calcium phosphate nanostructured surfaces offer a promising pathway for controlling key bioengineering processes such as cell cycle regulation, gene transfer, and patterned cell growth. The ionic dissolution products from these materials are known to affect, for example, the cell cycle of cells responsible for bone tissue formation. This effect may be genetically mediated as expression of various families of genes has been shown to be upregulated by Ca2+ ions. These include genes for cell-signaling molecules, growth factors, DNA synthesis and repair, and extracellular matrix proteins. Among the many nanoscale physical characteristics of calcium phosphates that may influence cell activity, its crystallographic texture is one of the least investigated. Yet, crystallographic texture is one of the essential microstructural features that determine the properties of polycrystals. Textures have been extensively studied in metals, geological materials, and selected ceramics. Preferred orientations in biologically relevant ceramics have received little attention. Naturally occurring bioceramics such as calcium phosphates often exhibit preferred orientations resulting from highly specific biological processes. These textures
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